HP Offers 3 levels of certification in NNM (NNM1,NNM2 and NNM3). This article talks about clearing NNM-1 exam, which is coded as HP-0632.

Golden Rules to clear your NNM-1 Exam:

Have good hands on experience on NNM

Brush up your OS skills and basic Networking skills, since there will be some questions about the same. (e.g. : what is the port for SNMP, What is ICMP etc.) There will be minimum 2 to 3 questions like this.

Have you attended any training from HP for NNM ? If so, they must have given you two books. Go through them, they are very good.